Scientific hypotheses are always open to refutation. The hypothesis "all crows are black" can be disproved if a white crow is found. However many black crows are found, the hypothesis can still be disproved by a single bird.

In reality the discovery of the white crow would be treated with scepticism. Those loyal to the black crow hypothesis would argue that it is a well known fact that all crows are black, and this has been shown by numerous studies. Therefore, the white crow is a hoax, forgery or some other kind of white bird. In short, an extraordinary claim. A comment in a birdwatchers notebook may be sufficient proof that a crow is black, but the white crow will have to be studied intensely to prove that it is truly white, and genuinely a crow. The sceptics would probably not be convinced until lots of white crows had been found and studied by scientists. Most scientists would be reluctant to have anything to do with the so called white crow, for fear of damaging their reputations

For example, tree trunks fluctuate with the phases of the moon. This was shown recently by a paper in Nature
(Ernst Zurcher, Maria-Giulia Cantiani, Francesco Sorbetti-Guerri and Denis Michel (1998) Tree stem diameters fluctuate with tide, Nature 392 16th April p.665). This particular extraordinary claim was nothing new. The experiment (requiring a calendar and tape measure) had been done several times before. Ridiculed and dismissed every time. The results were no surprise in the French timber industry, where timber is marked with a crescent or circle to indicate when it was cut